Numerous methods are known in the art for the reduction of triphenylphosphine oxide to triphenylphosphine, all of which entail either expense, complex reaction mechanisms, reagent handling problems, or other problems. Examples of such methods are the use of inorganic and organic silicone compounds with tertiary amines (U.S. Pat. No. 3,261,871), and the use of metals or metal aluminum hydrides in the presence of a silicone halide (U.S. Pat. No. 3,280,195). Other processes involve the use of a halo complexing agent to form an adduct of triphenylphosphine oxide, which is then heated to decomposition temperature to form the triphenylphosphine dihalide, which is subsequently reduced with a metal reducing agent (U.S. Pat. No. 3,405,180) or with iron with an oxidation potential of 0.44 volt (U.S. Pat. No. 3,780,111). In view of the complexity of the above methods, a simplified and inexpensive process which will yield a product of high purity is highly desirable.
The reducing agent diisobutylaluminum hydride is described in a sales brochure entitled "Specialty Reducing Agents," prepared by Texas Alkyls, Inc. (available through Stauffer Chemical Company, Westport, Conneticut 06880), published in April 1971. A table on page 15 of the brochure lists anticipated yields of various reduction reactions using diisobutylaluminum hydride. Among the entries in the table is a figure representing the triphenylphosphine oxide reaction. Subsequent to the publication of the brochure, actual experiments were run. The results, which are shown hereinbelow, prove that the quoted figure for the triphenylphosphine oxide reduction is unobtainable at the indicated reactant ratio. In thus teaching results which are unobtainable, as the data hereinbelow will show, the brochure is an inoperative disclosure of this particular use of the reagent. Furthermore, the description in the brochure indicates no preference between aliphatic and aromatic solvents, beyond stating that for many reductions, an aromatic solvent is preferred to insure the solubility of the species to be reduced (page 6 of the brochure).